linux-kernel - Re: [PATCH 1/2] locking/lockref: Use try_cmpxchg64 in CMPXCHG

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Message-ID: <CAHk-=wjd-RVttYwPc9RYD+x0b=WNYc_PZ2JKwPxc8fm54t0d2w@mail.gmail.com>
Date:   Thu, 26 May 2022 09:52:04 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Heiko Carstens <hca@...ux.ibm.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, Waiman.Long@...com,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 1/2] locking/lockref: Use try_cmpxchg64 in CMPXCHG_LOOP macro

On Thu, May 26, 2022 at 5:15 AM Michael Ellerman <mpe@...erman.id.au> wrote:
>
> Do you know of a benchmark that shows it up? I tried a few things but
> couldn't get lockref_get() to count for more than 1-2%.

Heh. 1% for a small instruction sequence that is only handful of
instructions and is used in just a couple of places counts as "very
hot" for me.

I assume you did the natural thing: threaded pathname lookup (with
paths being of the longer variety to not be dominated by system call
etc costs).

               Linus